Optical transmission system for radio access and high frequency optical transmitter

ABSTRACT

Modulators respectively modulate baseband signals into IF signals having different frequencies. Multiplexers multiplex the IF signals. A local oscillation signal source outputs a predetermined local oscillation signal. An external modulator intensity-modulates an optical signal using the local oscillation signal. An optical branching portion branches the intensity-modulated optical signal. Modulators respectively modulate the multiplexed IF signals onto the branched optical signals and then output the optical signals to radio base stations. Optical-electrical converters convert the optical signals into electric signals and antennas transmit the electrical signals to subscriber terminals.

This application is a divisional application of Ser. No. 09/892,605,filed Jun. 28, 2001 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an optical transmissionsystem for radio access and a high frequency optical transmitter, andmore particularly, to an optical transmission system for connecting, inradio access for coupling a center station and a plurality of subscriberterminals with a radio signal (a high frequency radio signal in amicrowave band, a millimeter wave band, or the like), the center stationand a radio base station by an optical fiber, and a high frequencyoptical transmitter used for the system.

2. Description of the Background Art

FIG. 17 illustrates an example of the configuration of a conventionaloptical transmission system used for radio access for connecting acenter station and a subscriber terminal through a radio base stationfor transmitting and receiving a radio signal.

The conventional optical transmission system shown in FIG. 17 isconstructed by respectively connecting a center station 600 to aplurality of radio base stations 701 to 70 n (n is an integer of notless than two; which is the same in the following present specification)through a plurality of downstream (from the center station to the radiobase stations) optical fibers 801 to 80 n. The center station 600includes a plurality of electrical-optical converters 611 to 61 nrespectively corresponding to the plurality of radio base stations 701to 70 n. The radio base stations 701 to 70 n respectively includeoptical-electrical converters 711 to 71 n, modulators 721 to 72 n,frequency converters 731 to 73 n, local oscillation signal sources 741to 74 n, and antennas 751 to 75 n. The operation of the conventionaloptical transmission system will be described.

In the center station 600, information to be transmitted to thesubscriber terminal through the radio base station 70 k is inputted inthe form of a baseband signal to an input terminal 6 k (k=1 to n; whichis the same in the following present specification). Theelectrical-optical converter 61 k converts the baseband signal inputtedfrom the input terminal 6 k into an optical signal. The optical signaloutputted from the electrical-optical converter 61 k is transmitted tothe radio base station 70 k from the center station 600 through thedownstream optical fiber 80 k.

In the radio base station 70 k, the optical signal transmitted from thecenter station 600 is inputted to the optical-electrical converter 71 k.The optical-electrical converter 71 k converts the inputted opticalsignal into an electric signal. The modulator 72 k converts the electricsignal obtained by the conversion into a signal having an intermediatefrequency (an IF signal). The local oscillation signal source 74 koutputs a local oscillation signal having a predetermined frequency. Thefrequency converter 73 k receives the IF signal and the localoscillation signal, and converts the IF signal into a signal having aradio frequency (an RF signal) using the local oscillation signal. TheRF signal is released to space through the antenna 75 k.

FIG. 18 shows, in a case where the center station 600 and the pluralityof radio base stations 701 to 707 are connected to each other, theconcept of a service area covered by each of the radio base stations.Areas 901 to 907 shown in FIG. 18 represent service areas respectivelycovered by the radio base stations 701 to 707.

Light signals respectively having different information are respectivelytransmitted to the radio base stations 701 to 707 from the centerstation 600 through the downstream optical fibers 801 to 807. In orderto avoid interference between the adjacent service areas, the pluralityof radio base stations 701 to 707 respectively change the frequencies oflocal oscillation signals outputted from the local oscillation signalsources 741 to 747 provided therein and convert IF signals into RFsignals having different frequencies (fd1 to fd7), to perform radiotransmission to the subscriber terminals. With respect to the radio basestations respectively covering the service areas which are not adjacentto each other (which correspond to the radio base stations 701, 705, and706 in the example of FIG. 18), the same radio frequency (fd1=fd5=fd6)may be set.

However, when the different information are optically transmitted fromthe center station 600 to the plurality of radio base stations 701 to 70n and then to many subscriber terminals, as shown in FIGS. 17 and 18,various problems arise as follows.

The first problem is that the electrical-optical converters 611 to 61 n,whose number (=n) corresponds to the number of the radio base stations701 to 70 n, are required in the center station 600.

The second problem is that the expensive frequency converters 731 to 73n for frequency-converting the IF signals into the RF signals arerespectively required in the plurality of radio base stations 701 to 70n.

The third problem is that when information to be transmitted to a lot ofsubscriber terminals are transmitted upon being time-divisionmultiplexed, a multiplexer is required in the center station 600. Inthis case, separators are respectively required in the radio basestations 701 to 70 n, and high-speed modulation processing is requiredfor each of the modulators 721 to 72 n.

The fourth problem is that when the capacity of the one radio basestation 70 k (the amount of information which can be transmitted fromthe antenna 75 k to the subscriber terminal) is increased for a newsubscriber terminal, each of components other than the antenna 75 k mustbe additionally installed in the radio base station 70 k, and amultiplexer for multiplexing the RF signals is also required.Particularly when the position of the subscriber terminal to be added isthe position where a substantially unobstructed line-of-sightpropagation path cannot be ensured from the existing radio base station70 k, the components shown in FIG. 17 must be all newly installed suchthat the line-of-sight propagation path can be ensured.

The fifth problem is that the frequencies of the local oscillationsignals outputted by the local oscillation signal source 74 k in theradio base station 70 k must be made to differ in order to avoid theinterference between the adjacent service areas. Therefore, differentcomponents or different adjustments (if with the same components) arerequired for each radio base station 70 k.

FIG. 19 illustrates the configuration of another conventional opticaltransmission system in which the configuration of each of radio basestations 701 to 70 n is simplified.

The conventional optical transmission system shown in FIG. 19 isconstructed by respectively connecting a center station 600 and theplurality of radio base stations 701 to 70 n through a plurality ofdownstream optical fibers 801 to 80 n. The center station 600respectively include modulators 621 to 62 n, frequency converters 631 to63 n, local oscillation signal sources 641 to 64 n, external modulators651 to 65 n, and light sources 661 to 66 n so as to correspond to theradio base stations 701 to 70 n. The radio base stations 701 to 70 nrespectively include optical-electrical converters 711 to 71 n andantennas 751 to 75 n. The operation of the conventional opticaltransmission system will be described.

In the center station 600, information to be transmitted to the radiobase station 70 k is inputted in the form of a baseband signal to aninput terminal 6 k. The modulator 62 k modulates the baseband signalinputted from the input terminal 6 k to an IF signal. The localoscillation signal source 64 k outputs a local oscillation signal havinga predetermined frequency. The frequency converter 63 k receives the IFsignal obtained by the modulation in the modulator 62 k and the localoscillation signal outputted from the local oscillation signal source 64k, and frequency-converts the IF signal into an RF signal using thelocal oscillation signal. The light source 66 k generates an opticalsignal having a predetermined wavelength. The external modulator 65 kreceives the RF signal obtained by the conversion in the frequencyconverter 63 k and the optical signal outputted from the light source 66k, and intensity-modulates the optical signal using the RF signal. Theintensity-modulated optical signal is transmitted to the radio basestation 70 k through the downstream optical fiber 80 k.

The optical signal transmitted from the center station 600 is inputtedto the radio base station 70 k upon propagating through the downstreamoptical fiber 80 k.

In the radio base station 70 k, the optical-electrical converter 71 kconverts the inputted optical signal into an electric signal, to outputan RF signal. The outputted RF signal is released into space from theantenna portion 75 k to the subscriber terminal as a radio signal.

In the conventional optical transmission system, therefore, the IFsignal is frequency-converted into the RF signal in the center station600. Accordingly, the radio base stations 701 to 70 n respectivelyrequire only the optical-electrical converters 711 to 71 n in additionto the antenna portions 751 to 75 n. Therefore, the conventional lighttransmission system has the effect of miniaturizing each of the radiobase stations 701 to 70 n.

In the configuration of the other conventional optical transmissionsystem shown in FIG. 19, however, the frequency converters 631 to 63 n,the external modulators 651 to 65 n, and the optical-electricalconverters 711 to 71 n must be high frequency devices (active devices)respectively operating in radio frequency bands. Such high-frequencydevices are generally expensive. In such a configuration that the centerstation 600 manages the plurality of radio base stations 701 to 70 n asin the other conventional optical transmission system, therefore, nexpensive devices are required, so that the entire system becomes veryexpensive.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide an opticaltransmission system for radio access and a high frequency opticaltransmitter in which the problems above are solved by so constructingthat transmission signals directed toward a plurality of radio basestations are collectively frequency-converted in a center station andoptically performing frequency-conversion from an IF signal to an RFsignal (further frequency conversion from an RF signal into an IFsignal) through an optical transmission path.

The present invention has the following features to attain the aboveobject.

A first aspect of the present invention is directed to an opticaltransmission system, used for radio access for transmitting informationbetween a center station and a subscriber terminal through a radio basestation for transmitting and receiving a radio signal to and from anantenna, for optically transmitting radio signals bidirectionally byrespectively connecting a plurality of radio base stations coveringdifferent service areas and the center station through a plurality ofoptical fibers, wherein

the center station includes at least

an electrical-optical converter, receiving one or more baseband signalsas one or more modulated electric signals each having a predeterminedintermediate frequency, for converting the electric signals into opticalsignals by intensity modulation,

a local oscillation signal source for outputting a predetermined localoscillation signal,

an external modulator for intensity-modulating the optical signalobtained by the conversion in the electrical-optical converter using thelocal oscillation signal outputted from the local oscillation signalsource, and

an optical branching portion for branching the optical signalintensity-modulated by the external modulator, and respectivelyoutputting optical signals obtained by the branching to the plurality ofoptical fibers, and

each of the plurality of radio base stations includes at least

an optical-electrical converter for converting the optical signaltransmitted through the optical fiber into an electric signal in a radiofrequency band, and

a band pass filter for extracting only an electric signal component in adesired frequency band from the electric signal obtained by theconversion in the optical-electrical converter, and feeding theextracted electric signal component to the antenna.

According to the first aspect, with respect to a downstream system, theelectrical-optical converter can be thus shared, and the externalmodulator for performing optical frequency conversion can be sharedbecause no electrical frequency converter is required. Further, thesignals can be multiplexed more easily, thereby making it possible tomore easily increase the transmission capacity with the increasingnumber of subscriber terminals, as compared with the conventionaloptical transmission system.

The center station may include at least

a light source for outputting predetermined light,

a local oscillation signal source for outputting a predetermined localoscillation signal,

an external modulator for intensity-modulating the light outputted fromthe light source using the local oscillation signal outputted from thelocal oscillation signal source,

an optical branching portion for branching an optical signal obtained bythe intensity modulation in the external modulator into optical signalswhose number corresponds to the number of the plurality of radio basestations, and

a plurality of IF modulators, receiving one or more modulated electricsignals each having a predetermined intermediate frequency by one ormore baseband signals for each of the radio base stations to which theelectric signal is to be transmitted, for respectivelyintensity-modulating the optical signals obtained by the branching inthe optical branching portion using the electric signals, andrespectively outputting the modulated optical signals to the pluralityof optical fibers, and

each of the plurality of radio base stations may include at least anoptical-electrical converter for converting the optical signaltransmitted through the optical fiber into an electric signal in a radiofrequency band, and feeding the electric signal to the antenna.

In such a configuration, with respect to a downstream system, theexternal converter for performing optical frequency conversion can beshared because no electrical frequency converter is required. Further,the signals can be multiplexed more easily, thereby making it possibleto more easily increase the transmission capacity with the increasingnumber of subscriber terminals, as compared with the conventionaloptical transmission system. In addition, no band filter is required.Therefore, radio base stations having the same configuration can beinstalled even when service areas respectively covered by the radio basestations differ.

The center station may include at least

an electrical-optical converter, receiving one or more modulatedelectric signals each having a predetermined intermediate frequency byone or more baseband signals for converting the electric signals intooptical signals by intensity modulation,

a local oscillation signal source for outputting a predetermined localoscillation signal,

a first external modulator for intensity-modulating the optical signalobtained by the conversion in the electrical-optical converter using thelocal oscillation signal outputted from the local oscillation signalsource,

a first optical branching portion for branching the optical signalintensity-modulated by the first external modulator, and respectivelyoutputting optical signals obtained by the branching to a plurality ofdownstream optical fibers,

a plurality of first optical-electrical converters for respectivelyconverting the optical signals transmitted from the plurality of radiobase stations through a plurality of upstream optical fibers intoelectric signals in intermediate frequency bands, and

a plurality of demodulators for respectively demodulating the electricsignals obtained by the conversion in the plurality of firstoptical-electrical converters to the baseband signals, and

each of the plurality of radio base stations may include at least

a second optical branching portion for branching the optical signaltransmitted through the downstream optical fiber into two opticalsignals,

a second optical-electrical converter for converting one of the opticalsignals obtained by the branching in the second optical branchingportion into an electric signal in a radio frequency band,

a band pass filter for extracting only an electric signal component in adesired frequency band from the electric signal obtained by theconversion in the second optical-electrical converter,

a circulator for outputting the electric signal component extracted bythe band pass filter and the radio signal received by the antenna,respectively, to the antenna and a second external modulator, and

the second external modulator for intensity-modulating the other opticalsignal obtained by the branching in the second optical branching portionusing the radio signal outputted from the circulator, and outputting theintensity-modulated optical signal to the upstream optical fiber.

In such a configuration, with respect to both upstream and downstreamsystems, the electrical-optical converter can be shared, and theexternal modulator for performing optical frequency conversion can beshared because no electrical frequency converter is required. Further,the signals can be multiplexed more easily, thereby making it possibleto more easily increase the transmission capacity with the increasingnumber of subscriber terminals, as compared with the conventionaloptical transmission system.

The center station may include at least

a light source for outputting predetermined light,

a local oscillation signal source for outputting a predetermined localoscillation signal,

a first external modulator for intensity-modulating the light outputtedfrom the light source using the local oscillation signal outputted fromthe local oscillation signal source,

a first optical branching portion for branching an optical signalobtained by the intensity modulation in the first external modulatorinto optical signals whose number corresponds to the number of theplurality of radio base stations,

a plurality of IF modulators, receiving one or more modulated electricsignals each having a predetermined intermediate frequency by one ormore baseband signals for each of the radio base stations to which theelectric signal is to be transmitted, for respectivelyintensity-modulating the optical signals obtained by the branching inthe first optical branching portion using the electric signals, andrespectively outputting the modulated optical signals to the pluralityof downstream optical fibers, and

a plurality of first optical-electrical converters for respectivelyconverting the optical signals transmitted from the plurality of radiobase stations through a plurality of upstream optical fibers intoelectric signals in intermediate frequency bands, and

a plurality of demodulators for respectively demodulating the electricsignals obtained by the conversion in the plurality of firstoptical-electrical converters to the baseband signals, and

each of the plurality of radio base stations may include

a second optical branching portion for branching the optical signaltransmitted through the downstream optical fiber into two opticalsignals,

a second optical-electrical converter for converting one of the opticalsignals obtained by the branching in the second optical branchingportion into an electric signal in a radio frequency band,

a circulator for outputting the electric signal obtained by theconversion in the second optical-electrical converter and the radiosignal received by the antenna, respectively, to the antenna and asecond external modulator, and

the second external modulator for intensity-modulating the other opticalsignal obtained by the branching in the second optical branching portionusing the radio signal outputted from the circulator, and outputting theintensity-modulated optical signal to the upstream optical fiber.

In such a configuration, with respect to both upstream and downstreamsystems, the external modulator for performing optical frequencyconversion can be shared because no electric frequency converter isrequired. Further, the signals can be multiplexed more easily, therebymaking it possible to more easily increase the transmission capacitywith the increasing number of subscriber terminals, as compared with theconventional optical transmission system. In addition, no band filter isrequired. Therefore, radio base stations having the same configurationcan be installed even when the service areas respectively covered by theradio base stations differ.

Preferably, a downstream system through which the optical signal istransmitted by radio from the radio base station to the subscriberterminal and an upstream system through which the optical signal istransmitted by radio from the subscriber terminal to the radio basestation are made to differ in a radio frequency to be used.Consequently, the radio signals may not interfere with each otherbetween the upstream system and the downstream system.

Preferably, the frequencies of the radio signals respectively used inthe radio base stations are set so as to differ, or the frequencies ofthe radio signals used in the radio base stations which cover theadjacent service areas are set to differ from each other. Consequently,the radio signals may not interfere with each other between the adjacentservice areas.

Preferably, the optical signal outputted from the external modulator isan optical single-sideband signal with a carrier and a single-sidebandcomponent. Alternatively, a Mach-Zehnder type external modulator is usedfor the external modulator, and a bias point in the external modulatoris set to a point at which light output power is the minimum or maximumso that the optical signal is intensity-modulated by a component whichis twice the frequency of the local oscillation signal. Consequently, itis possible to avoid the decrease in the level of the electric signalafter optical-electrical conversion which has conventionally occurredwhen the optical fiber has dispersion characteristics. Further, in thelatter case, the oscillation frequency of the local oscillation signalmay be one-half of that in the conventional example, and the operationfrequencies of the local oscillation signal source and the externalmodulator may be one-half of that in the conventional example.

Preferably, a semiconductor laser for converting an electric signal intoan optical signal through direct modulation is used for theelectrical-optical converter. Consequently, the multiplexed electricsignals in intermediate frequency bands can be converted into theoptical signals through direct modulation, thereby making it possible toperform the electrical-optical conversion simply and at low cost.

More preferably, an optical fiber in which the wavelength of the opticalsignal outputted from the electrical-optical converter and the zerodispersion wavelength almost coincide with each other is used for theoptical fiber. Consequently, the wavelength of the optical signal andthe zero dispersion wavelength of the optical fiber can almost coincidewith each other. Accordingly, distortion induced by the dispersion canbe avoided, thereby making it possible to realize high-qualitytransmission.

A second aspect of the present invention is directed to an opticaltransmission system, used for radio access for transmitting informationbetween a center station and a subscriber terminal through a radio basestation for transmitting and receiving a radio signal to and from anantenna, for optically transmitting radio signals bidirectionally byrespectively connecting first to n-th radio base stations coveringdifferent service areas and the center station through first to n-thupstream and downstream optical fibers respectively provided so as tocorrespond to the radio base stations, wherein

the center station includes

first to n-th electrical-optical converters for respectively convertingone or more signals each having a predetermined intermediate frequencyinto first to n-th optical signals having different wavelengths λd1 toλdn uniquely corresponding to the first to n-th radio base stations,

a wavelength multiplexer for multiplexing the first to n-th opticalsignals obtained by the conversion,

a local oscillation signal source for outputting a local oscillationsignal having a predetermined frequency,

an optical modulator for intensity-modulating the multiplexed opticalsignals outputted from the wavelength multiplexer using the localoscillation signal, and

a wavelength separator for wavelength-separating the multiplexed opticalsignals intensity-modulated into first to n-th modulated optical signalshaving wavelengths λd1 to λdn, and sending out the k-th modulatedoptical signal to the k-th downstream optical fiber, and

the k-th radio base station includes an optical-electrical converter,receiving the k-th modulated optical signal having the wavelength λdktransmitted through the k-th downstream optical fiber, for convertingthe modulated optical signal into an electric signal in a radiofrequency band, and outputting the electric signal.

According to the second aspect, the optical signalswavelength-multiplexed are collectively externally modulated tofrequency-convert the signals in the intermediate frequency bands intothe signals in the radio frequency bands. Therefore, the electricalfrequency converter, which has been conventionally required, is notrequired, and the optical modulator for performing the optical frequencyconversion can be shared among the plurality of radio base stations.Further, the signals in the radio frequency bands to be transmitted tothe plurality of radio base stations are separated from each other inthe light wavelength region, thereby making it possible to easilyseparate the signals even if the radio frequencies radiated from theplurality of radio base stations are the same.

In this case, the center station may include

first to n-th electrical-optical converters for respectively convertingone or more signals each having a predetermined intermediate frequencyinto first to n-th downstream optical signals having differentwavelengths λd1 to λdn uniquely corresponding to the first to n-th radiobase stations,

first to n-th upstream light sources respectively outputting first ton-th upstream optical signals having wavelengths λu1 to λun which differfrom any of the wavelengths λd1 to λdn and differ from one another,

a wavelength multiplexer for multiplexing the first to n-th downstreamoptical signals obtained by the conversion and the outputted first ton-th upstream optical signals,

a local oscillation signal source for outputting a local oscillationsignal having a predetermined frequency,

an optical modulator for intensity-modulating the multiplexed opticalsignals outputted from the wavelength multiplexer using the localoscillation signal,

a wavelength separator for wavelength-separating the multiplexed opticalsignals intensity-modulated to the first to n-th modulated downstreamoptical signals having the wavelengths λ1 to λdn and the first to n-thmodulated upstream optical signals having the wavelengths λu1 to λun,and sending out the k-th modulated downstream optical signal, togetherwith the k-th modulated upstream optical signal, to the k-th downstreamoptical fiber, and

first to n-th optical-electrical converters for respectively convertingthe optical signals transmitted through the first to n-th upstreamoptical fibers into electric signals, and

the k-th radio base station may include

a two-wavelength separator, receiving the optical signal transmittedthrough the k-th downstream optical fiber, for separating the opticalsignal into the k-th modulated downstream optical signal having thewavelength λdk and the k-th modulated upstream optical signal having thewavelength λuk,

an optical-electrical converter for converting the k-th modulateddownstream optical signal obtained by the separation in thetwo-wavelength separator into an electric signal and outputting theelectric signal, and

an RF modulator for intensity-modulating the k-th modulated upstreamoptical signal obtained by the separation in the two-wavelengthseparator using the inputted radio signal, and sending out the k-thmodulated upstream optical signal intensity-modulated to the k-thupstream optical fiber.

In such a configuration, the first to n-th light sources for outputtinglight having different wavelengths for transmitting an upstream signalare provided, and the upstream optical signal and a downstream opticalsignal are wavelength-multiplexed, so that the optical modulator can beshared among the plurality of radio base stations for optical modulationof not only the downstream optical signal but also the upstream opticalsignal with the local oscillation signal. Further, the upstream opticalsignal modulated using the local oscillation signal isintensity-modulated using the radio signal received by the antenna, sothat mixing of the local oscillation signal and the radio signal isperformed in a light region. Accordingly, the radio base station canalso have a frequency conversion function for converting the radiosignal into the signal having an intermediate frequency.

Preferably, the wavelengths λd1 to λdn of the first to n-th downstreamoptical signals are set so as to belong to a predetermined firstwavelength band,

the wavelengths λu1 to λun of the first to n-th upstream optical signalsare set so as to belong to a predetermined second wavelength band,

the second wavelength separator in the k-th radio base stationwavelength-separates the optical signal transmitted through the k-thdownstream optical fiber into a signal in the first wavelength band anda signal in the second wavelength band, to separate the optical signalinto the k-th modulated downstream optical signal having the wavelengthλdk and the k-th modulated upstream optical signal having the wavelengthλuk.

The wavelengths of the optical signals outputted from the first to n-thelectrical-optical converters and the wavelengths of the optical signalsoutputted from the first to n-th light sources are set so as torespectively belong to wavelength bands in definite ranges, therebymaking it possible to easily separate the modulated downstream opticalsignal and the modulated upstream optical signal in the two-wavelengthseparator in the radio base station.

On the other hand, the center station may include

first to n-th electrical-optical converters for respectively convertingone or more signals each having a predetermined intermediate frequencyinto first to n-th downstream optical signals having differentwavelengths λd1 to λdn, which belong to a predetermined first wavelengthband, uniquely corresponding to the first to n-th radio base stations,

first to n-th upstream light sources respectively outputting first ton-th upstream optical signals having wavelengths λu1 to λun which differfrom any of the wavelengths λd1 to λdn and belong to a predeterminedsecond wavelength band,

a wavelength multiplexer for multiplexing the first to n-th downstreamoptical signals obtained by the conversion and the outputted first ton-th upstream optical signals,

a local oscillation signal source for outputting a local oscillationsignal having a predetermined frequency,

an optical modulator for intensity-modulating the multiplexed opticalsignals outputted from the wavelength multiplexer using the localoscillation signal,

a wavelength separator for wavelength-separating the multiplexed opticalsignals intensity-modulated into first to n-th modulated downstreamoptical signals having the wavelengths λd1 to λdn and the first to n-thmodulated upstream optical signals having the wavelengths λu1 to λun,and sending out the k-th modulated downstream optical signal, togetherwith the k-th modulated upstream optical signal, to the k-th downstreamoptical fiber, and

first to n-th optical-electrical converters for respectively convertingthe optical signals transmitted through the first to n-th upstreamoptical fibers into electric signals, and

the k-th radio base station may include an electro-absorption typemodulator, receiving the optical signal transmitted through the k-thdownstream optical fiber to separate the optical signal into the k-thmodulated downstream optical signal having the wavelength λdk and thek-th modulated upstream optical signal having the wavelength λuk,converting the k-th modulated downstream optical signal in the firstwavelength band representing an optical-electrical conversion functioninto an electric signal and outputting the electric signal, andintensity-modulating the k-th modulated upstream optical signal in thesecond wavelength band representing an electrical-optical conversionfunction using the inputted radio signal and sending out the k-thmodulated upstream optical signal intensity-modulated to the k-thupstream optical fiber.

In such a configuration, the wavelengths of the optical signalsoutputted from the first to n-th electrical-optical converters and thewavelengths of the optical signals outputted from the first to n-thlight sources are suitably set, and the electro-absorption typemodulator for performing optical-electrical conversion andelectrical-optical conversion is installed in the radio base stationdepending on the wavelength of the inputted optical signal in place ofthe two-wavelength separator, the optical-electrical converter, and theRF modulator, described above. Consequently, it is possible tosignificantly simplify the configuration of the radio base station inaddition to the effect obtained by the above-mentioned configuration.

Preferably, the first to n-th upstream light sources respectively outputthe first to n-th upstream optical signals which uniquely correspond tothe first to n-th downstream optical signals and have wavelengths λu1 toλun respectively different from the wavelengths λd1 to λdn of the firstto n-th downstream optical signals by predetermined amounts fs.Consequently, it is possible to simply separate the wavelengths λdk andλuk by using only an n output wavelength separator for separating noptical signals multiplexed at equal spacing in the configuration of thewavelength separator.

A third aspect of the present invention is directed to a high frequencyoptical transmitter, used in a center station connected to a pluralityof radio base stations respectively covering different service areasusing a plurality of optical fibers, for optically transmitting radiosignals, characterized by comprising

a three-branching portion for branching an inputted electric signal intofirst and second electric signals which are the same in phase and athird electric signal which has a phase difference of 90° to the firstand second electric signals;

an electrical-optical converter for converting the third electric signalinto a light intensity modulated signal;

a first delay controller for adjusting the propagation time of the firstelectric signal;

a second delay controller for adjusting the propagation time of thesecond electric signal;

a two-branching portion for branching an inputted local oscillationsignal into first and second local oscillation signals which areopposite in phase;

a third delay controller for adjusting the propagation time of the firstlocal oscillation signal;

a fourth delay controller for adjusting the propagation time of thesecond local oscillation signal;

a first multiplexer for multiplexing the first electric signal outputtedfrom the first delay controller and the first local oscillation signaloutputted from the third delay controller;

a second multiplexer for multiplexing the second electric signaloutputted from the second delay controller and the second localoscillation signal outputted from the fourth delay controller; and

a differential intensity modulator, having first and second electrodes,for modulating the light-intensity modulated signal outputted from theelectrical-optical converter by respectively inputting signals obtainedby the multiplexing in the first and second multiplexers to the firstand second electrodes,

the first to fourth delay controllers being adjusted such that the firstand second electric signals inputted to the first and second electrodesof the differential intensity modulator through the first and secondmultiplexers are the same in phase, to subject the optical signaloutputted from the electrical-optical converter to phase modulation andsubject the optical signal to optical modulation which is the same inamount as and is opposite in phase to the frequency deviation (an FMindex) of alight frequency modulation component of the optical signal.

As described above, according to the third aspect, the optical signaloutputted from the electrical-optical converter is phase-modulated usinga part of the electric signal inputted to the electrical-opticalconverter utilizing the external modulator for performing frequencyconversion. Consequently, the light frequency modulation component(wavelength chirping) which occurs at the time of the electrical-opticalconversion can be canceled without using an additional opticalcomponent, and distortion due to wavelength dispersion which is inducedby the function of the light frequency modulation and the wavelengthdispersion characteristics of the optical fiber can be suppressed,thereby making it possible to realize high-performance opticaltransmission.

A high frequency optical transmitter may include

a three-branching portion for branching an inputted electric signal intofirst and second electric signals which are the same in phase and athird electric signal which has a phase difference of 90° from the firstand second electric signals;

an electrical-optical converter for converting the third electric signalinto a light intensity modulated signal;

a first delay controller for adjusting the propagation time of the firstelectric signal;

a second delay controller for adjusting the propagation time of thesecond electric signal;

a two-branching portion for branching an inputted local oscillationsignal into first and second local oscillation signals which have adifference of 90° to each other;

a third delay controller for adjusting the propagation time of the firstlocal oscillation signal;

a fourth delay controller for adjusting the propagation time of thesecond local oscillation signal;

a first multiplexer for multiplexing the first electric signal outputtedfrom the first delay controller and the first local oscillation signaloutputted from the third delay controller;

a second multiplexer for multiplexing the second electric signaloutputted from the second delay controller and the second localoscillation signal outputted from the fourth delay controller; and

a differential intensity modulator, having first and second electrodes,for modulating the light-intensity modulated signal outputted from theelectrical-optical converter by respectively inputting signals obtainedby the multiplexing in the first and second multiplexers to the firstand second electrodes,

the first and second delay controllers being adjusted such that a phasedifference between the first and second electric signals inputted to thefirst and second electrodes of the differential intensity modulatorthrough the first and second multiplexers is zero, to subject theoptical signal outputted from the electrical-optical converter to phasemodulation and subject the optical signal to optical modulation which isthe same in amount as and is opposite in phase to the frequencydeviation of a light frequency modulation component of the opticalsignal, and

the third and fourth delay controllers being adjusted such that thefirst and second local oscillation signals inputted to the first andsecond electrodes of the differential intensity modulator through thefirst and second multiplexers have a difference of 90° to each other, tosubject the optical signal to optical side-band modulation with a lightcarrier.

In such a configuration, the optical single-sideband modulation isperformed, thereby making it possible to avoid the problem that an RFsignal component obtained by frequency-converting the electric signal isgreatly decreased because an upper sideband and a lower sideband of thelight-intensity modulation component are canceled by the effect ofwavelength dispersion which occurs in a case where the optical signalwhich has been subjected to non-differential external modulation(optical double-sideband modulation) is subjected to optical-electricalconversion after being transmitted a long distance.

Furthermore, a high frequency optical transmitter may include

a two-branching portion for branching an inputted electric signal intofirst and second electric signals which have a difference of 90° to eachother;

an electrical-optical converter for converting the first electric signalinto a light intensity modulated signal;

a delay controller for adjusting the propagation time of the secondelectric signal; and

an integrated modulator, comprising a phase modulator and an intensitymodulator formed on the same substrate, for modulating the lightintensity modulated signal outputted from the electrical-opticalconverter by inputting the second electric signal outputted from thedelay controller to the phase modulator and inputting an inputted localoscillation signal to the intensity modulator,

in the phase modulator, the optical signal outputted from theelectrical-optical converter being subjected to phase modulation andsubjected to optical modulation which is opposite in phase to thefrequency deviation of a light frequency modulation component of theoptical signal.

In such a configuration, it is possible to easily make a delayadjustment for canceling the light frequency modulation component byintegrating the phase modulator for canceling the light frequencymodulation component and the intensity modulator for performingfrequency conversion. Further, the electric signal inputted to the phasemodulator and the local oscillation signal inputted to the intensitymodulator need not be multiplexed, thereby making it possible to furtherreduce the power of the electric signal inputted to the intensitymodulator.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of an opticaltransmission system according to a first embodiment of the presentinvention;

FIG. 2 is a block diagram showing a configuration in a case where asubscriber terminal is added in the optical transmission systemaccording to the first embodiment of the present invention;

FIG. 3 is a block diagram showing the configuration of an opticaltransmission system according to a second embodiment of the presentinvention;

FIG. 4 is a block diagram showing the configuration of an opticaltransmission system according to a third embodiment of the presentinvention;

FIG. 5 shows diagrams each showing an example of a signal spectrumprocessed by the optical transmission system according to the thirdembodiment of the present invention;

FIG. 6 is a block diagram showing the configuration of an opticaltransmission system according to a fourth embodiment of the presentinvention;

FIG. 7 is a block diagram showing the configuration of an opticaltransmission system according to a fifth embodiment of the presentinvention;

FIG. 8 is a block diagram showing the configuration of an opticaltransmission system according to a sixth embodiment of the presentinvention;

FIG. 9 shows diagrams each for explaining an example of a wavelengthseparating operation performed in a wavelength separator 460;

FIG. 10 is a block diagram showing another example of the configurationsof radio base stations 210 to 20 n shown in FIG. 8;

FIG. 11 is a diagram showing an example of optical-electrical conversionefficiency and electrical-optical conversion efficiency obtained in afield effect absorption type modulator 29 k shown in FIG. 10;

FIG. 12 is a block diagram showing the configuration of a high frequencyoptical transmitter according to a seventh embodiment of the presentinvention;

FIG. 13 is a block diagram showing the specific configuration of adifferential light-intensity modulator 550 shown in FIG. 12;

FIG. 14 shows diagrams each illustrating an example of a light spectrummeasured using the high frequency optical transmitter according to theseventh embodiment of the present invention;

FIG. 15 shows diagrams each showing the difference between light spectrain a case where different external modulation schemes are used in thehigh frequency optical transmitter according to the seventh embodimentof the present invention;

FIG. 16 is a block diagram showing the configuration of a high frequencyoptical transmitter according to an eighth embodiment of the presentinvention;

FIG. 17 is a block diagram showing the configuration of a conventionaloptical transmission system;

FIG. 18 is a diagram showing the concept of service areas 901 to 907respectively covered by a plurality of radio base stations 701 to 707;and

FIG. 19 is a block diagram showing the configuration of anotherconventional optical transmission system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a first embodiment ofthe present invention. In FIG. 1, the optical transmission systemaccording to the first embodiment is so constructed that a centerstation 100 and a plurality of radio base stations 201 to 20 n arerespectively connected to each other through a plurality of downstreamoptical fibers 301 to 30 n.

The center station 100 includes a plurality of modulators 110, amultiplexer 120, an electrical-optical converter 130, a localoscillation signal source 140, an external modulator 150, and an opticalbranching portion 160. The radio base stations 201 to 20 n respectivelyinclude optical-electrical converters 211 to 21 n, band filters 221 to22 n, and antennas 231 to 23 n. The operation of the opticaltransmission system according to the first embodiment will be described.

In the center station 100, different information to be transmitted tosubscriber terminals are respectively inputted to input terminals 1 inthe form of baseband signals. The plurality of modulators 110respectively modulate the baseband signals inputted from the inputterminals 1 to IF signals having different predetermined frequencies.The frequencies are respectively determined on the basis of thefrequencies of radio signals to be transmitted to the subscriberterminals from the radio base stations 201 to 20 n. The multiplexer 120multiplexes the plurality of IF signals outputted from the plurality ofmodulators 110. The electrical-optical converter 130 converts the IFsignals multiplexed by the multiplexer 120 to an optical signal throughintensity modulation. The local oscillation signal source 140 outputs alocal oscillation signal having a predetermined frequency. The frequencyof the local oscillation signal is determined on the basis of themodulation frequencies of the modulators 110 and the frequencies of theradio signals. The external modulator 150 receives the optical signalobtained by the conversion in the electrical-optical converter 130 andthe local oscillation signal outputted from the local oscillation signalsource 140, and intensity-modulates the optical signal using the localoscillation signal. The optical branching portion 160 branches theoptical signal intensity-modulated by the external modulator 150 intooptical signals, whose number corresponds to the number (=n) of theradio base stations, and outputs the optical signals, respectively, tothe radio base stations 201 to 20 n through the downstream opticalfibers 301 to 30 n.

The optical signals outputted from the center station 100 arerespectively inputted to the radio base stations 201 to 20 n afterpropagating through the downstream optical fibers 301 to 30 n. In theradio base station 20 k, the optical-electrical converter 21 k convertsthe inputted optical signal into an electric signal. By the conversion,an RF signal obtained by frequency-converting the IF signal can beobtained. The reason for this is that in the electrical-opticalconverter 130 and the external modulator 150 in the center station 100,intensity modulation is doubly performed using the IF signal and thelocal oscillation signal. The band filter 22 k receives the RF signaloutputted from the optical-electrical converter 21 k, and extracts onlyan RF signal component having a desired radio frequency therefrom. Theextracted RF signal component is released to space from the antenna 23 kto the subscriber terminal.

In the present invention, the intensity modulation is thus doublyperformed in the center station 100. In each of the radio base stations201 to 20 n, therefore, the RF signal obtained by frequency-convertingthe IF signal can be obtained by only converting the optical signalwhich has propagated into the electric signal. Consequently, theelectrical-optical converter 130 and the external modulator 150 forperforming frequency conversion can be shared among the plurality ofradio base stations 201 to 20 n. Consequently, the number of componentsin each of the radio base stations 201 to 20 n can be more significantlyreduced, as compared with that in the conventional example. Further, itis easy to carry and install, for example, each of the radio basestations 201 to 20 n.

Furthermore, the center station 100 frequency-division multiplexes theinformation to be transmitted to the plurality of subscriber terminalsin the multiplexer 120 for analog optical transmission. In the centerstation 100, therefore, the modulators 110, whose number corresponds tothe number of information to be multiplexed, are prepared. In theoptical transmission system according to the present invention,therefore, no separator/multiplexer and high-speed modulator arerequired, unlike the case where information to be transmitted istime-division multiplexed for transmission as described in theBackground Art section.

Furthermore, in the present invention, the intensity modulation isdoubly performed. Accordingly, it is possible to use a semiconductorlaser, which is applicable only to a relatively low frequency signal butis superior in distortion characteristics to the external modulator, forintensity modulation of the frequency-division multiplexed IF signals(performed by the electrical-optical converter 130). Also, it ispossible to use an external modulator which operates to a high frequencyfor intensity modulation of the local oscillation signal (performed bythe external modulator 150). Further, the frequency of the localoscillation signal is relatively high. Accordingly, an externalmodulator which has been matched in a particular high frequency band formodulation can also be used for the intensity modulation of the localoscillation signal. Further, a Mach-Zehnder type external modulator canalso be used for the intensity modulation of the local oscillationsignal. When the Mach-Zehnder type external modulator is used, a biaspoint is set to a point at which light output power is the maximum orthe minimum, thereby making it possible to perform optical frequencyconversion at a frequency which is twice the frequency of the localoscillation signal. A method of setting the bias point cannot be usedfor the intensity modulation of the IF signals frequency-divisionmultiplexed because a second-order distortion component is increased.However, the method can be used for the intensity modulation of thelocal oscillation signal because the local oscillation signal is of onlyone carrier. Further, in this method, the frequency of the localoscillation signal used for the intensity modulation may be a frequencywhich is one-half of that in the normal frequency conversion.Accordingly, it is possible to use a low-cost local oscillation signalsource 140.

On the other hand, a single mode fiber (SMF) having a zero dispersionwavelength at 1.3 μm is generally used for an optical fiber used inoptical transmission, for example. Generally when the optical fiber islaid, not only an actually required number of optical fibers, but alsopreliminary optical fibers which are currently not used, aresimultaneously laid in consideration of future use. Consequently, anunused SMF which has already been laid may be thus used as thedownstream optical fiber 30 k in the present invention. Further, when anoptical amplifier must be used in order to compensate for a branchingloss and a transmission loss, a wavelength of 1.5 μm is used as thewavelength of a light source used for the electrical-optical converter130. At this time, when double-sideband modulation (DSB modulation) isused as a modulation scheme carried out in the external modulator 150,and a high frequency signal in a microwave band or a millimeter waveband is optically transmitted by the SMF, the power of the highfrequency signal after the optical transmission may be greatly decreaseddepending on the transmission distance due to the effect of thedispersion of the SMF. In order to avoid the effect of the dispersion,an optical single-sided band modulation (optical SSB modulation) with acarrier and a single-sideband component may be used for the modulationscheme carried out in the external modulator 150. Further, externalmodulation may be performed using a Mach-Zehnder type external modulatoras the external modulator 150 and by setting a point at which lightoutput power is the minimum or the maximum to a bias point.

In a case where the downstream optical fiber 30(n+1) is newly laid, forexample, when the zero dispersion wavelength of the downstream opticalfiber 30(n+1) and the wavelength of light outputted from theelectrical-optical converter 130 can be selected, it is desired thatboth the wavelengths be selected to coincide with each other. In thiscase, even if the DSB modulation is used as a modulation scheme, it isnot affected by the dispersion of the SMF. Further, in theelectrical-optical converter 130, when the frequency-divisionmultiplexed IF signals are converted into the optical signal throughdirect modulation, distortion due to dispersion corresponding to thefrequency after the frequency conversion is induced irrespective of theexternal modulation scheme of the local oscillation signal. From thepoint of view of preventing the distortion due to dispersion from beinginduced, it is desired that the zero dispersion wavelength of thedownstream optical fiber 30(n+1) and the wavelength of the lightoutputted from the electrical-optical converter 130 coincide with eachother.

Description is made of a case where a subscriber terminal is added toincrease the transmission capacity in the optical transmission system.FIG. 2 is a block diagram showing a configuration in a case where thesubscriber terminal is added in the optical transmission systemaccording to the first embodiment of the present invention.

As described above, the center station 100 frequency-divisionmultiplexes information to be transmitted to a plurality of subscriberterminals. As can be seen from comparison between FIG. 1 and FIG. 2,when the subscriber terminal is added, therefore, only a correspondingadditional modulator 101 is required to be added to the center station100, to input its output signal to the multiplexer 120. Consequently, anIF signal outputted from the additional modulator 101 isfrequency-division multiplexed and is optically transmitted to all theradio base stations 201 to 20 n. Accordingly, the corresponding radiobase station converts the optically transmitted optical signal into anRF signal, and then extracts the RF signal having a desired radiofrequency through the band filter 22 k. Therefore, the present inventionshows that it is more simply feasible to increase the transmissioncapacity as the new subscriber terminal is added, as compared with thatin the Background Art section.

When the position of the subscriber terminal to be added is a positionwhere a line-of-sight propagation path cannot be ensured from theexisting radio base station, if there is room for the number of branchesby the optical branching portion 160, the downstream optical fiber30(n+1) and a new radio base station 20(n+1) may be additionallyinstalled, thereby making it possible to more significantly reduce thenumber of components which must be additionally installed, as comparedwith that in the above-mentioned Background Art.

As described above, in the optical transmission system according to thefirst embodiment of the present invention, the IF signal obtained byfrequency-division multiplexing the plurality of IF signals is convertedinto an optical signal by intensity modulation, and the optical signalis externally modulated using the local oscillation signal, therebycollectively frequency-converting the plurality of IF signals into theRF signals in an optical signal state. Information to be transmitted tothe subscriber terminals are optically transmitted in the form of the RFsignals from the center station 100 to the plurality of radio basestations 201 to 20 n. Consequently, the following effect is obtained inthe present invention.

First, the plurality of IF signals are optically transmitted upon beingfrequency-division multiplexed in the center station 100. Accordingly,the electrical-optical converter 130 can be shared among the pluralityof radio base stations 201 to 20 n. Second, the frequency conversion isoptically performed by the external modulator 150 in the center station100. Accordingly, the electrical frequency converter which is anindispensable component in the Background Art is not required in each ofthe radio base stations 210 to 20 n, and the external modulator 150 canbe shared among the radio base stations 201 to 20 n. Third, unlike theconventional configuration, the modulator having a multiplexer/separatorand a high-speed modulation function is not required in the presentinvention. Fourth, the capacity can be easily increased as newsubscriber terminals are added, thereby making it possible to provide anoptical transmission system superior in expandability.

Second Embodiment

In the optical transmission system according to the first embodiment,the plurality of IF signals are multiplexed by the frequency divisionmultiplexing, and the IF signals frequency-division multiplexed aredoubly intensity-modulated. In the radio base stations 201 to 20 n,therefore, the band filters 221 to 22 n must be respectively used toextract RF signal components having desired radio frequencies from thetransmitted RF signals.

In the second embodiment, description is made of an optical transmissionsystem which does not respectively require the band filters 221 to 22 nin the configurations of radio base stations 201 to 20 n.

FIG. 3 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a second embodiment ofthe present invention. In FIG. 3, the optical transmission systemaccording to the second embodiment is so constructed that a centerstation 100 and a plurality of radio base stations 201 to 20 n arerespectively connected to each other through a plurality of downstreamoptical fibers 301 to 30 n.

The center station 100 includes a plurality of modulators 110, aplurality of multiplexers 121 to 12 n, a plurality of IF modulators 181to 18 n, a light source 170, a local oscillation signal source 140, anexternal modulator 150, and an optical branching portion 160. The radiobase stations 201 to 20 n respectively include optical-electricalconverters 211 to 21 n and antennas 231 to 23 n. The operation of theoptical transmission system according to the second embodiment will bedescribed below.

In the center station 100, different information to be transmitted tosubscriber terminals are respectively inputted to input terminals 1 inthe form of baseband signals. The plurality of modulators 110respectively modulate the baseband signals inputted from the inputterminals 1 to IF signals having different predetermined frequencies.The frequencies are respectively determined on the basis of thefrequencies of radio signals transmitted to the subscriber terminalsfrom the radio base stations 201 to 20 n. Each of the multiplexers 121to 12 n multiplexes the plurality of IF signals outputted from theplurality of modulators 110 for each of the radio base stations 201 to20 n. The light source 170 generates an optical signal having apredetermined wavelength. The local oscillation signal source 140outputs a local oscillation signal having a predetermined frequency. Thefrequency of the local oscillation signal is determined on the basis ofthe modulation frequencies of the modulators 110 and the frequencies ofthe radio signals. The external modulator 150 receives the opticalsignal outputted from the light source 170 and the local oscillationsignal outputted from the local oscillation signal source 140, andintensity-modulates the optical signal using the local oscillationsignal. The optical branching portion 160 branches the optical signalintensity-modulated by the external modulator 150 into optical signals,whose number corresponds to the number (=n) of the radio base stations,and respectively outputs the optical signals to the plurality of IFmodulators 181 to 18 n. Each of the IF modulators 181 to 18 n receivesthe optical signal obtained by the branching and an IF signal obtainedby the multiplexing, and intensity-modulates the optical signaldepending on the IF signal. The IF modulators 181 to 18 n respectivelycorrespond to the radio base stations 201 to 20 n. Each of the IFmodulators 181 to 18 n intensity-modulates the optical signal dependingon the IF signal such that only an RF signal component used in a servicearea covered by the corresponding radio base station is transmitted. Theoptical signals intensity-modulated by the IF modulators 181 to 18 n arerespectively transmitted to the radio base stations 201 to 20 n throughthe downstream optical fibers 301 to 30 n.

The optical signals outputted from the center station 100 arerespectively inputted to the radio base stations 201 to 20 n afterpropagating through the downstream optical fibers 301 to 30 n. In theradio base station 20 k, the optical-electrical converter 21 k convertsthe inputted optical signal into an electric signal. By the conversion,an RF signal obtained by frequency-converting the IF signal and having adesired radio frequency can be obtained. The reason for this is that inthe external modulator 150 and the IF modulator 18 k in the centerstation 100, intensity modulation is doubly performed using the localoscillation signal and the IF signal. The RF signal obtained by theconversion is released to space from the antenna 23 k to the subscriberterminal.

As described above, in the optical transmission system according to thesecond embodiment of the present invention, the IF modulators 181 to 18n are installed in the center station 100 for the radio base stations201 to 20 n, and only the RF signal component used in the service areacovered by each of the radio base stations 201 to 20 n is transmitted tothe radio base station. In the radio base station 20 k, therefore, radiotransmission can be performed at different frequencies for the adjacentservice areas without using the band filter 22 k for extracting the RFsignal component having a desired radio frequency, as described in thefirst embodiment. In the configuration, optical transmission is possibleeven at the same frequency. Further, the band filter 22 k is not usedfor the radio base station 20 k. Accordingly, the radio base stationfrom which the desired radio frequency can be outputted need not beselected and used for each of the service areas. Alternatively, theradio base station need not be adjusted such that the desired radiofrequency can be outputted for each of the service areas (that is, inthe service areas, the radio base stations having the same configurationcan be used). Consequently, it is feasible to reduce the cost of theoptical transmission system.

Third Embodiment

The first and second embodiments were characterized by a case whereinformation is transmitted from the center station to the subscriberterminal (in a downstream direction). Accordingly, description was madeof the optical transmission system comprising the configuration for onlydownstream transmission.

In the third embodiment, description is then made of an opticaltransmission system in which an RF signal in an optical signal stateobtained by frequency-converting an IF signal is utilized for a casewhere information is transmitted from a subscriber terminal to a centerstation 100 (in an upstream direction), to simplify a configurationrequired for upstream transmission.

FIG. 4 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a third embodiment ofthe present invention. In FIG. 4, the optical transmission systemaccording to the third embodiment is so constructed that a centerstation 100 and a plurality of radio base stations 201 to 20 n arerespectively connected to each other through a plurality of downstreamoptical fibers 301 to 30 n and a plurality of upstream optical fibers351 to 35 n.

The center station 100 includes a plurality of modulators 110, aplurality of demodulators 102, a multiplexer 120, an electrical-opticalconverter 130, a local oscillation signal source 140, an externalmodulator 150, an optical branching portion 160, and a plurality ofoptical-electrical converters 191 to 19 n. The radio base stations 201to 20 n respectively include optical branching portions 261 to 26 n,optical-electrical converters 211 to 21 n, external modulators 251 to 25n, band filters 221 to 22 n, circulators 241 to 24 n, and antennas 231to 23 n. The operation of the optical transmission system according tothe third embodiment will be described below.

Description is now made of the downstream transmission from the centerstation 100 to each of the radio base stations 201 to 20 n.

Processing from the time when a plurality of baseband signals arerespectively inputted to input terminals 1 in the center station 100 tothe time when optical signals are respectively outputted to the radiobase stations 201 to 20 n after propagating through downstream opticalfibers 301 to 30 n in the downstream transmission is the same as theprocessing described in the first embodiment and hence, the descriptionthereof is not repeated.

In the radio base station 20 k, the optical branching portion 26 kbranches the inputted optical signal into two optical signals, andoutputs one of the optical signals obtained by the branching and theother optical signal, respectively, to the optical-electrical converter21 k and the external modulator 25 k. The optical-electrical converter21 k converts the optical signal obtained by the branching and outputtedfrom the optical branching portion 26 k into an electric signal. By theconversion, an RF signal obtained by frequency-converting an IF signalcan be obtained. The band filter 22 k receives the RF signal outputtedfrom the optical-electrical converter 21 k, and extracts only an RFsignal component having a desired radio frequency. The extracted RFsignal component is released to space from the antenna 23 k to thesubscriber terminal through the circulator 24 k.

Description is now made of upstream transmission from each of the radiobase stations 201 to 20 n to the center station 100.

An RF signal transmitted from the subscriber terminal is received by theantenna 23 k. The received RF signal is outputted to the externalmodulator 25 k through the circulator 24 k. In the present invention,the circulator 24 k is thus provided between the optical-electricalconverter 21 k and the antenna 23 k, so that the antenna 23 k is sharedbetween the upstream transmission and the downstream transmission. Theexternal modulator 25 k receives the optical signal obtained by thebranching and outputted from the optical branching portion 26 k and theRF signal received by the antenna 23 k, and intensity-modulates theoptical signal by the RF signal. The optical signal intensity-modulatedby the external modulator 25 k is outputted to the center station 100through the upstream optical fiber 35 k.

The optical signals respectively outputted from the radio base stations201 to 20 n are inputted to the center station 100 after respectivelypropagating through the upstream optical fibers 351 to 35 n. In thecenter station 100, each of the optical-electrical converters 191 to 19n converts the inputted optical signal into an electric signal. By theconversion, an IF signal obtained by frequency-converting an RF signalcan be obtained. The demodulators 102 respectively demodulate the IFsignals obtained by the conversion in the optical-electrical converters191 to 19 n to baseband signals, and output the baseband signals fromoutput terminals 2.

In the radio base station 20 k, the optical signal obtained by thebranching and outputted to the external modulator 25 k isintensity-modulated using the local oscillation signal generated by thelocal oscillation signal source 140 in the external modulator 150provided in the center station 100. In the external modulator 25 k,therefore, the optical signal obtained by the branching and outputted isintensity-modulated using the RF signal received by the antenna 23 k, sothat the intensity modulation is doubly performed using the localoscillation signal and the RF signal. Accordingly, the optical signalpropagating from the external modulator 25 k in the radio base station20 k is converted into an electric signal by the optical-electricalconverter 19 k in the center station 100, thereby making it possible toobtain the IF signal obtained by frequency-converting the RF signal. Inorder to avoid the interference between an upstream signal transmittedfrom the subscriber terminal and a downstream signal transmitted fromthe center station 100, the frequencies of the RF signals used in therespective directions must be made to differ.

As described above, in the present invention, the downstream opticalsignal which has already been intensity-modulated using the localoscillation signal is branched into two optical signals using theoptical branching portion 26 k installed in the radio base station 20 k,and one of the optical signals is utilized as the upstream opticalsignal inputted to the external modulator 25 k. In the externalmodulator 25 k, therefore, the RF signal transmitted from the subscriberterminal is optically frequency-converted into the IF signal, therebymaking it possible to install the local oscillation signal source 140and the modulator 110 in the center station 100. As a result, in thepresent invention, each of the radio base stations 201 to 20 n can beminiaturized.

Furthermore, processing performed in the optical transmission systemaccording to the third embodiment will be specifically described withreference to FIG. 5.

(a) of FIG. 5 is a diagram showing an example of the spectrum of thedownstream signal outputted from the optical-electrical converter 21 k.In this example, it is indicated that an IF signal 1 (frequency:f_(IF1)) is converted into a downstream RF signal 1 (frequency: f_(RF1))by an LO signal (frequency: f_(LO)). (b) of FIG. 5 is a diagram showingan example of the spectrum of the upstream signal received by theantenna 23 k. In this example, it is indicated that an upstream RFsignal 2 (frequency: f_(RF2)) having a frequency difference of Δf fromthe downstream RF signal 1 is received by the antenna 23 k. In theexternal modulator 25 k, the optical signal transmitted from the centerstation 100 is intensity-modulated using the RF signal 2, so that mixingof the signals having the spectrums shown in FIGS. 5( a) and 5(b) isperformed in the state of light. A signal spectrum in a case where anoptical signal obtained by the mixing is converted into an electricsignal in the optical-electrical converter 19 k is as illustrated in (c)of FIG. 5. That is, an IF signal 2 having a frequency f_(IF2) isobtained by mixing the LO signal and the RF signal 2 in the externalmodulator 25 k. Consequently, only a component having the frequencyf_(IF2) is separated by the band filter, thereby making it possible toextract only the IF signal 2 obtained by down-converting the RF signal2. At this time, the frequency difference between the IF signal 1 andthe IF signal 2 is Δf.

The frequency of the upstream RF signal 2 is separated from thefrequency of the downstream RF signal 1 by Δf, so that the upstreamsignal and the downstream signal can be down-converted into signalshaving low frequencies without being adversely affected by each other.

As described above, the optical transmission system according to thethird embodiment of the present invention also has the effect ofminiaturizing each of the radio base stations 201 to 20 n with respectto the upstream system in addition to the effect obtained with respectto the downstream system described in the first embodiment, so that itis easily installed outdoors.

In the first to third embodiments, the plurality of modulators 110 areprovided in order to respectively modulate the inputted baseband signalsinto the plurality of IF signals having different frequencies. When thebaseband signal may be only modulated to the IF signal having a singlefrequency, however, the one modulator 110 may be provided. In this case,the IF signal outputted from the modulator 110 may be directly inputtedto the electrical-optical converter 130 without providing themultiplexer 120.

Fourth Embodiment

FIG. 6 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a fourth embodiment ofthe present invention. In FIG. 6, in the optical transmission systemaccording to the fourth embodiment, a center station 100 and a pluralityof radio base stations 201 to 20 n are respectively connected to eachother through a plurality of downstream optical fibers 301 to 30 n.

The center station 100 includes a plurality of modulators 111 to 11 nand a plurality of electrical-optical converters 131 to 13 nrespectively corresponding to the plurality of radio base stations 201to 20 n, a wavelength multiplexer 430, a local oscillation signal source140, an optical modulator 450, and a wavelength separator 460. The radiobase stations 201 to 20 n respectively include optical-electricalconverters 211 to 21 n and antennas 231 to 23 n. The operation of theoptical transmission system according to the fourth embodiment will bedescribed.

In the center station 100, information to be transmitted to the radiobase station 20 k is inputted to an input terminal 1 k in the form of abaseband signal. The modulator 11 k modulates the baseband signalinputted from the input terminal 1 k into an IF signal having apredetermined frequency. The frequency is determined on the basis of thefrequency of a radio signal transmitted to a subscriber terminal fromthe radio base station 20 k. The electrical-optical converter 13 kconverts the IF signal obtained by the modulation in the modulator 11 kinto an optical signal through direct modulation. The wavelengths of theoptical signals obtained by the conversion in the electrical-opticalconverters 131 to 13 n are previously assigned so as to differ. Theelectrical-optical converters 131 to 13 n are set such that thewavelengths of the optical signals are equally spaced, for example. Thewavelength multiplexer 430 wavelength-multiplexes the optical signalshaving the different wavelengths respectively outputted from theelectrical-optical converters 131 to 13 n. The local oscillation signalsource 140 outputs a local oscillation signal having a predeterminedfrequency f_(LO). The frequency f_(L0) of the local oscillation signalis determined on the basis of the modulation frequencies of themodulators 111 to 11 n and the frequencies of the radio signals to berespectively transmitted from the radio base stations 201 to 20 n to thesubscriber terminals. The optical modulator 450 receives the opticalsignals wavelength-multiplexed in the wavelength multiplexer 430 and thelocal oscillation signal outputted from the local oscillation signalsource 140, to collectively intensity-modulate thewavelength-multiplexed optical signals using the local oscillationsignal. By the intensity modulation, it is possible to obtain theoptical signal having an RF component obtained by optically convertingthe IF signal. The wavelength separator 460 separates the opticalsignals intensity-modulated by the optical modulator 450 into aplurality of optical signals depending on the wavelengths, andrespectively transmits the corresponding optical signals to the radiobase stations 201 to 20 n through the downstream optical fibers 301 to30 n.

The optical signals transmitted from the center station 100 arerespectively inputted to the radio base stations 201 to 20 n afterpropagating through the downstream optical fibers 301 to 30 n.

In the radio base station 20 k, the optical-electrical converter 21 kconverts the inputted optical signal into an electric signal, to outputan RF signal. By the conversion, an RF signal obtained byfrequency-converting an IF signal can be obtained. The outputted RFsignal is released to space from the antenna 23 k to the subscriberterminal as a radio signal after only its desired frequency component isextracted therefrom.

As described above, in the optical transmission system according to thefourth embodiment of the present invention, only the IF signals havingfrequencies which are in close proximity to or identical to each otherare respectively converted into light intensity modulated signals havingdifferent wavelengths. After the light intensity modulated signals arewavelength-multiplexed, they are collectively intensity-modulated usingthe local oscillation signal through external modulation. Since theplurality of IF signals are collectively optically frequency-convertedinto the RF signals, therefore, no electrical frequency converter isrequired, and the optical modulator for performing optical frequencyconversion can be shared among the plurality of radio base stations,thereby making it possible to realize low-cost frequency conversion.

Furthermore, the optical signals which have been collectivelyintensity-modulated are respectively optically transmitted to the radiobase stations even after being separated depending on the wavelengths.Even when the frequencies of the RF signals are the same, therefore, theRF signals can be transmitted without interfering with each other. Inaddition, the optical signals can be easily separated in a lightwavelength region, thereby making it possible to provide a low-costoptical transmission system for radio access.

Although in the fourth embodiment, description was made of such aconfiguration that the wavelength separator 460 is installed in thecenter station 100, the wavelength separator 460 need not be necessarilyinstalled in the center station 100. For example, the wavelengthseparator 460 may be installed in separated fashion as an independentrelay station, or may be installed in each of the radio base stations201 to 220 n. In the latter case, however, such a configuration that theoptical signal outputted from the optical modulator 450 is branched intooptical signals, and the optical signals are respectively outputted tothe radio base stations 201 to 20 n must be newly provided in the centerstation 100.

Although description was made of a case where the IF signals inputted tothe electrical-optical converters 131 to 13 n are of one channel,multi-channel IF signals may be respectively inputted thereto, in whichcase the same effect is obtained. In this case, the IF signals on thechannels may be inputted to the electrical-optical converters 131 to 13n after being frequency-division multiplexed.

Furthermore, in the fourth embodiment, the intensity modulation isdoubly performed. Accordingly, it is possible to use a semiconductorlaser which is only applicable to a relatively low frequency signal butis superior in distortion characteristics to an external modulator andan external modulator which operates to a high frequency, respectively,for intensity modulation of the IF signals wavelength-multiplexed (theelectrical-optical converters 131 to 13 n) and intensity modulation ofthe local oscillation signal (the optical modulator 450). Further, thefrequency of the local oscillation signal is relatively high.Accordingly, an external modulator which has been matched in aparticular high frequency band for modulation can be also used for theintensity modulation of the local oscillation signal. Further, it isalso possible to use a Mach-Zehnder type external modulator for theintensity modulation of the local oscillation signal. When theMach-Zehnder type external modulator is used, a bias point is set to apoint at which light output power is the maximum or the minimum, therebymaking it possible to perform optical frequency conversion at afrequency which is twice the frequency of the local oscillation signal.A method of setting the bias point cannot be used for the intensitymodulation of the IF signals frequency-division multiplexed because asecond-order distortion component is increased. However, it can be usedfor the intensity modulation of the local oscillation signal because thelocal oscillation signal is of only one carrier. Further, in thismethod, the frequency of the local oscillation signal used for theintensity modulation may be the frequency which is one-half of that inthe normal frequency conversion. Accordingly, it is possible to use as alow-cost local oscillation signal source 140 and a low-cost opticalmodulator 450.

When an optical amplifier must be used in order to compensate for abranching loss and a transmission loss, a wavelength of 1.5 μm is usedas the wavelength of a light source used for the electrical-opticalconverters 131 to 13 n. At this time, the optical fiber which hasalready been laid is an SMF. When DSB modulation is used as a modulationscheme carried out in the optical modulator 450, and a high frequencysignal in a microwave band or a millimeter wave band is opticallytransmitted by the SMF, the power of the high frequency signal after theoptical transmission may be greatly reduced depending on thetransmission distance by the effect of the dispersion of the SMF. Inorder to avoid the effect of the dispersion, optical SSB modulation witha carrier and a sideband component may be used for a modulation schemecarried out in the optical modulator 450. Accordingly, a Mach-Zehndertype external modulator may be used as the optical modulator 450, and apoint at which light output power is the minimum or the maximum may beset to a bias point, to perform external modulation.

Fifth Embodiment

In the optical transmission system according to the fourth embodiment ofthe present invention, in the electrical-optical converters 131 to 13 n,the IF signals are respectively converted into the optical signalsthrough direct modulation. When the multi-channel IF signal is subjectedto electrical-optical conversion, therefore, wavelength chirping occurs.When there is wavelength dispersion of the downstream optical fibers 301to 30 n, distortion due to wavelength dispersion is induced, therebydegrading the transmission characteristics.

In the fifth embodiment, description is made of an optical transmissionsystem in which no wavelength chirping occurs.

FIG. 7 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a fifth embodiment ofthe present invention. In FIG. 7, in the optical transmission systemaccording to the fifth embodiment, a center station 100 and a pluralityof radio base stations 201 to 20 n are respectively connected to eachother through a plurality of downstream optical fibers 301 to 30 n.

The center station 100 includes a plurality of modulators 111 to 11 n, aplurality of light sources 171 to 17 n, and a plurality of IF modulators481 to 48 n which respectively correspond to the plurality of radio basestations 201 to 20 n, a wavelength multiplexer 430, a local oscillationsignal source 140, an optical modulator 450, and a wavelength separator460. The radio base stations 201 to 20 n respectively includeoptical-electrical converters 211 to 21 n and antennas 231 to 23 n. Inthe optical transmission system according to the fifth embodiment, thesame components as those in the optical transmission system according tothe fourth embodiment are assigned the same reference numerals andhence, the description thereof is not repeated. In the opticaltransmission system according to the fifth embodiment, description ismade centering on the operations of the different components.

The light sources 171 to 17 n respectively output optical signals havingdifferent wavelengths. For example, the light sources 171 to 17 nrespectively output optical signals such that the wavelengths areequally spaced. The IF modulator 48 k receives an IF signal modulated bythe modulator 11 k and the optical signal outputted from the lightsource 17 k, and intensity-modulates the optical signal using the IFsignal. The intensity-modulated optical signals arewavelength-multiplexed in the wavelength multiplexer 430. Thereafter,the multiplexed optical signals are respectively optically transmittedto the radio base stations 201 to 20 n after being subjected to theabove-mentioned processing.

As described above, in the optical transmission system according to thefifth embodiment shown in FIG. 7, the light sources 171 to 17 n and theIF modulators 48 a to 48 n are used to convert the IF signals intooptical signals through external modulation, so that no wavelengthchirping occurs. Even when there is wavelength dispersion of thedownstream optical fibers 301 to 30 n, therefore, the signals can betransmitted without degrading transmission characteristics. Theconfiguration as in the fifth embodiment is particularly useful for acase where the characteristics cannot be changed on the side of theoptical fiber, for example, a system is newly constructed using theoptical fiber which has already been laid.

Sixth Embodiment

The fourth and fifth embodiments were characterized by a case whereinformation is transmitted from the center station 100 to the subscriberterminals (in the downstream direction). Accordingly, description wasmade of the optical transmission system comprising a configuration foronly downstream transmission.

In the sixth embodiment, description is then made of an opticaltransmission system in which an RF signal in an optical signal stateobtained by frequency-converting an IF signal is utilized for a casewhere information is transmitted from a subscriber terminal to a centerstation 100 (in an upstream direction), to simplify a configurationrequired for upstream transmission.

FIG. 8 is a block diagram showing the configuration of an opticaltransmission system for radio access according to a sixth embodiment ofthe present invention. In FIG. 8, in the optical transmission systemaccording to the sixth embodiment, a center station 100 and a pluralityof radio base stations 201 to 20 n are respectively connected to eachother through a plurality of downstream optical fibers 301 to 30 n and aplurality of upstream optical fibers 351 to 35 n.

The center station 100 includes a plurality of modulators 111 to 11 n, aplurality of electrical-optical converters 131 to 13 n, a plurality ofupstream light sources 471 to 47 n, a plurality of optical-electricalconverters 421 to 42 n, and a plurality of demodulators 411 to 41 nrespectively corresponding to the plurality of radio base stations 201to 20 n, a wavelength multiplexer 431, a local oscillation signal source140, an optical modulator 450, and a wavelength separator 460. The radiobase stations 201 to 20 n respectively include two-wavelength separators271 to 27 n, RF modulators 281 to 28 n, optical-electrical converters211 to 21 n, circulators 241 to 24 n, and antennas 231 to 23 n. In theoptical transmission system according to the sixth embodiment, the samecomponents as those in the optical transmission system according to thefourth embodiment are assigned to the same reference numerals and hence,the description thereof is not repeated. The optical transmission systemaccording to the sixth embodiment will be described, centered on theoperations of the different components.

Description is now made of the downstream transmission from the centerstation 100 to each of the radio base stations 201 to 20 n.

The upstream light sources 471 to 47 n respectively output opticalsignals used for transmitting upstream signals from the radio basestations 201 to 20 n to the center station 100. Here, the upstream lightsources 471 to 47 n are set such that the wavelengths of the outputtedoptical signals differ from each other and also respectively differ fromwavelengths previously assigned to the electrical-optical converters 131to 13 n. The wavelength multiplexer 431 wavelength-multiplexes theoptical signals having the different wavelengths respectively outputtedfrom the electrical-optical converters 131 to 13 n and the opticalsignals having the different wavelengths respectively outputted from theupstream light sources 471 to 47 n. The optical modulator 450 receivesthe optical signals wavelength-multiplexed by the wavelength multiplexer431 and a local oscillation signal outputted from the local oscillationsignal source 140, and collectively intensity-modulates thewavelength-multiplexed optical signals using the local oscillationsignal. The wavelength separator 460 separates the optical signalsintensity-modulated by the optical modulator 450 into a plurality ofoptical signals depending on the wavelengths, and respectively transmitsthe corresponding optical signals to the radio base stations 201 to 20 nthrough the downstream optical fibers 301 to 30 n. That is, thewavelength separator 460 transmits the optical signal outputted from theelectrical-optical converter 13 k and the optical signal outputted fromthe upstream light source 47 k to the radio base station 20 k throughthe downstream optical fiber 30 k.

In the radio base station 20 k, the two-wavelength separator 27 kwavelength-separates the optical signal transmitted from the centerstation 100, and outputs the optical signal outputted from theelectrical-optical converter 13 k and the optical signal outputted fromthe upstream light source 47 k, respectively, to the optical-electricalconverter 21 k and the RF modulator 28 k. The optical-electricalconverter 21 k converts the optical signal wavelength-separated by thetwo-wavelength separator 27 k into an electric signal. By theconversion, an RF signal obtained by frequency-converting an IF signalcan be obtained. The RF signal is released to space from the antenna 23k to a subscriber terminal through the circulator 24 k.

Description is now made of the upstream transmission from each of theradio base stations 201 to 20 n to the center station 100.

An RF signal transmitted from the subscriber terminal is received by theantenna 23 k. The received RF signal is outputted to the RF modulator 28k through the circulator 24 k. In the present invention, therefore, thecirculator 24 k is provided between the optical-electrical converter 21k and the antenna 23 k, so that the antenna 23 k is shared between theupstream transmission and the downstream transmission. The RF modulator28 k receives the optical signal wavelength-separated by thetwo-wavelength separator 27 k and the RF signal received by the antenna23 k, and intensity-modulates the optical signal using the RF signal.The optical signal intensity-modulated by the RF modulator 28 k isoutputted to the center station 100 through the upstream optical fiber35 k.

The optical signal outputted from the radio base station 20 k isinputted to the center station 100 upon propagating through the upstreamoptical fiber 35 k.

In the center station 100, the optical-electrical converter 42 kconverts the inputted optical signal into an electric signal. By theconversion, an IF signal obtained by frequency-converting the RF signalcan be obtained. The demodulator 41 k demodulates the IF signal obtainedby the conversion in the optical-electrical converter 42 k to a basebandsignal, and outputs the baseband signal from an output terminal 4 k.

In the radio base station 20 k, the optical signal in the upstream lightsource 47 k which is outputted to the RF modulator 28 k isintensity-modulated using the local oscillation signal generated by thelocal oscillation signal source 140 in the optical modulator 450provided in the center station 100. In the RF modulator 28 k, therefore,the wavelength-separated optical signal is intensity-modulated using thereceived RF signal in the RF modulator 28 k, so that the optical signalis doubly intensity-modulated using the local oscillation signal and theRF signal. Accordingly, the optical signal propagating from the RFmodulator 28 k in the radio base station 20 k is received by theoptical-electrical converter 42 k in the center station 100 and isdetected, so that mixing of both the signals is performed, and the RFsignal is frequency-converted into the IF signal.

In the sixth embodiment, in order to transmit the radio signal receivedby the antenna 23 k, the optical signal outputted from the upstreamlight source 47 k, together with the optical signal outputted from theelectrical-optical converter 13 k, is intensity-modulated in the opticalmodulator 450, and is then used for the intensity modulation in the RFmodulator 28 k. Consequently, the RF signal is frequency-converted intothe IF signal in the optical-electrical converter 42 k, so that thenumber of devices for high-frequency signal processing can be reduced.

Referring now to FIG. 9, a wavelength separating operation performed inthe wavelength separator 460 in the center station 100 will be describedby taking a specific example.

An n output wavelength separator for separating n optical signalsmultiplexed at equal spacing in wavelength, for example, can be used forthe wavelength separator 460. It is possible to use, as the n outputwavelength separator, for example, an arrayed waveguide grating (AWG)separator introduced in “Wavelength Multiplexing Optical SemiconductorComponent” (written by Yoshikuni et al.) reported in a magazine “O plusE” published in November 1997. The n output wavelength separator havingan AWG structure has a periodic wavelength passband when it is viewedfrom one output terminal.

As shown in (a) of FIG. 9, when the n output wavelength separator isused, therefore, the wavelength λdk of the optical signal outputted fromthe electrical-optical converter 13 k and the wavelength λuk of theoptical signal outputted from the upstream light source 47 k arepreviously adjusted so as to coincide with the periodic wavelengthpassband of the corresponding output terminal. Consequently, the opticalsignals having the two different wavelengths λdk and λuk respectivelyoutputted from the electrical-optical converter 13 k and the upstreamlight source 47 k can be together taken out of the same output terminal.

On the other hand, not the above-mentioned n output wavelength separatorhaving the periodic wavelength passband but a separator having awavelength passband having a predetermined width can be also used as thewavelength separator 460 when it is viewed from one output terminal.When such a separator is used, as shown in (b) of FIG. 9, the wavelengthλdk of the optical signal outputted from the electrical-opticalconverter 13 k and the wavelength λuk of the optical signal outputtedfrom the upstream light source 47 k are previously adjusted so as to bein close proximity to each other within the wavelength passband of theoutput terminal. Consequently, the optical signals having the twodifferent wavelengths λdk and λuk respectively outputted from theelectrical-optical converter 13 k and the upstream light source 47 k canbe together taken out of the same output terminal.

Referring to FIGS. 10 and 11, another configuration used for each of theradio base stations 201 to 20 n will be described.

FIG. 10 is a block diagram showing the configuration of the radio basestation 20 k using an electro-absorption type modulator 29 k in place ofthe two-wavelength separator 27 k, the optical-electrical converter 21k, the circulator 24 k and the RF modulator 28 k in the radio basestation 20 k shown in FIG. 8. The electro-absorption type modulator 29 kis a device having both an optical-electrical conversion function and anelectrical-optical conversion function. Accordingly, the configurationof the radio base station 20 k can be simplified, as shown in FIG. 10.The electro-absorption type modulator 29 k is described in “Full-DuplexFiber-Optic RF Subcarrier Transmission Using a Dual-FunctionModulator/Demodulator” (Andreas Stöhr, et al.) reported in a document“IEEE Trans. Microwave Theory Tech. Vol. 47, No. 7” published in 1999,for example.

The optical-electrical conversion efficiency and the electrical-opticalconversion efficiency of the electro-absorption type modulator 29 k havewavelength dependency and are such characteristics that high efficiencyis obtained in different wavelength areas, as indicated by a dotted linein FIG. 11. Consequently, suitable wavelength setting is performed suchthat optical signals to be subjected to optical-electrical conversionwhich are outputted from the electrical-optical converters 131 to 13 nand optical signals to be subjected to electrical-optical conversionwhich are outputted from the upstream light sources 471 to 47 n arerespectively arranged on the side of a short wavelength and a longwavelength (FIG. 11), thereby making it possible to make effective useof the electro-absorption type modulator 29 k.

As described above, in the optical transmission system according to thesixth embodiment of the present invention, in order to transmit theradio signal received by the radio base station to the center station,the radio signal can be frequency-converted into the IF signal as anoptical signal state by wavelength-multiplexing a plurality ofunmodulated light having different wavelengths on a downstream opticalsignal and previously externally modulating the light signal using thelocal oscillation signal. Consequently, no electric frequency converteris required for the radio base station, and an optical modulator foroptically performing frequency conversion can be shared among theplurality of radio base stations. Further, the light source need not beinstalled in the radio base station, so that the optical transmissionsystem can be easily maintained. Further, the electro-absorption typemodulator is used for optical receiving and optical modulation, therebymaking it possible to simplify the configuration of the radio basestation.

Seventh Embodiment

As described above, in a center station 100 in an optical transmissionsystem according to the present invention, an inputted IF signal isconverted into an optical signal through direct modulation in anelectrical-optical converter (e.g., a semiconductor laser), and theoptical signal is further intensity-modulated again using a localoscillation signal in an external modulator.

Letting cos(ωt) be the IF signal, sin(ω0t) be the optical signal, m bethe degree of optical modulation at the time of direct modulation, β1 bea light frequency modulation index based on the IF signal, and βL0 be alight phase modulation index at the time of external modulation, thedoubly modulated optical signal (electric field representation; E(t)) isexpressed by the following equation (1):

$\begin{matrix}\begin{matrix}{{E(t)}\; = {\left. \sqrt{}\left\lbrack {\left\{ {1 + {J\; 1\;\left( {\beta\; L\; 0} \right)\;\cos\;\left( {{\omega L}\; 0\; t} \right)}} \right\}\;\left\{ {1 + {m\;\cos\;\left( {\omega\;\left( {t\; - \;{\tau\; 1}} \right)} \right)}} \right\}} \right\rbrack \right.*}} \\{\sin\left\lbrack {{\omega\; 0\; t} + {\beta\; 1\;\sin\;\left( {\omega\; t} \right)}} \right\rbrack} \\{\;{= \left. \sqrt{}\left\lbrack {1 + {m\;\cos\;\left( {\omega\;\left( {t\; - \;{\tau\; 1}} \right)} \right)}\; + {J\; 1\;\left( {\beta\; L\; 0} \right)\;\cos\;\left( {\omega\; L\; 0\; t} \right)} +}\; \right. \right.}} \\{\left. {m\; J\; 1\;\left( {\beta\; L\; 0} \right)\;{\cos\left( {\omega\; L\; 0\; t} \right)}\;{\cos\left( {\omega\left( {t - {\tau\; 1}} \right)} \right)}} \right\rbrack*} \\{\sin\left\lbrack {{\omega\; 0\; t} + {\beta\; 1\sin\left( {\omega\; t} \right)}} \right\rbrack}\end{matrix} & (1)\end{matrix}$

As apparent from the foregoing equation (1), the doubly modulatedoptical signal E(t) has a light-intensity modulation component obtainedby frequency-modulating a modulation frequency (ω) by theelectrical-optical converter by a modulation frequency (ωL0) by theexternal modulator.

Generally, the electrical-optical modulator, such as the semiconductorlaser, has lower distortion characteristics but has a relativelynarrower frequency band, as compared with the external modulator.Contrary to this, the external modulator has broad-band characteristicsbut has inferior distortion characteristics. Consequently, theelectrical-optical converter and the external modulator are connected incascade, thereby making it possible to make use of low distortioncharacteristics of the electrical-optical converter and wide bandcharacteristics of the external modulator. Therefore, it is possible torealize low distortion transmission of a high frequency signal.

In a case where the electrical-optical converter and the externalmodulator are simply connected in cascade, however, wavelengthdistortion is induced in an optical signal outputted from a light sourcefor direct modulation by the exertion of a light frequency modulationcomponent (wavelength chirping) and wavelength dispersioncharacteristics on the optical signal. Particularly, transmissioncharacteristics of the optical signal are greatly degraded at the timeof long-distance transmission.

In the seventh embodiment, therefore, description is made of a highfrequency optical transmitter, which is applicable to the center station100, in which a light frequency modulation component of a directlymodulated optical signal is suppressed, and transmission characteristicsare not degraded at the time of long-distance transmission.

FIG. 12 is a block diagram showing the configuration of a high frequencyoptical transmitter according to a seventh embodiment of the presentinvention. In FIG. 12, the high frequency optical transmitter accordingto the seventh embodiment includes a three-branching portion 510, firstto fourth delay controllers 521 to 524, first and second multiplexers531 and 532, an electrical-optical converter 540, a differentiallight-intensity modulator 550, a local oscillation signal source 560,and a two-branching portion 570.

The high-frequency optical transmitter is applicable in place of theconfigurations of the electrical-optical converter 130, the localoscillation signal source 140, and the external modulator 150 in each ofthe center stations 100 in the first and third embodiments, and isapplicable in place of the configurations of the light source 170, thelocal oscillation signal source 140, the external modulator 150, theoptical branching portion 160, and the corresponding IF modulator 18 kin the center station 100 in the second embodiment. The high frequencyoptical transmitter is applicable by inputting the signal obtained bymultiplexing the IF signals outputted from the plurality of modulators111 to 11 n to an IF input terminal 51, inputting the optical signaloutputted by the wavelength multiplexer 430 or 431 to the differentiallight-intensity modulator 550 as an optical signal outputted from theelectrical-optical converter 540, and inputting an output signal of anoutput terminal 54 to the wavelength separator 460 in the center station100 in each of the fourth to sixth embodiments. The operation of thehigh frequency optical transmitter according to the seventh embodimentof the present invention will be described.

An IF signal having an intermediate frequency inputted from the IF inputterminal 51 is branched into first to third IF signals in thethree-branching portion 510. The first and second IF signals and thethird IF signal are respectively inputted to the first and second delaycontrollers 521 and 522 and the electrical-optical converter 540. Thefirst and second IF signals are set so as to be the same in phase anddiffer in phase by 90° from the third IF signal.

The third IF signal is converted into an optical signal by directmodulation in the electrical-optical converter 540 and is outputted. Atthis time, the directly modulated optical signal outputted from theelectrical-optical converter 540 is subjected to light intensitymodulation as well as light frequency modulation, and its electric fieldrepresentation: ELD(t) is given by the following equation (2), lettingcos(ωt) be the IF signal, sin(ω0t) be the optical signal, m be thedegree of optical modulation, and β1 be a frequency modulation indexbased on the IF signal:ELD(t)=√{square root over ( )}[1+m cos(ωt)] sin [ω0t+β1 sin (ωt)]  (2)

On the other hand, the first IF signal is outputted to one terminal ofthe first multiplexer 531 after the amount of propagation delay isadjusted to a predetermined value by the first delay controller 521.Similarly, the second IF signal is outputted to one terminal of thesecond multiplexer 532 after the amount of propagation delay is adjustedto a predetermined value by the second delay controller 522.

A local oscillation signal outputted from the local oscillation signalsource 560 is branched into first and second local oscillation signalswhich differ in phase by 180° in the two-branching portion 570. Thefirst local oscillation signal is outputted to the other terminal of thefirst multiplexer 531 after the amount of propagation delay is adjustedin the third delay controller 523 such that it is equal in propagationtime to the first IF signal. Further, the second local oscillationsignal is outputted to the other terminal of the second multiplexer 532after the amount of propagation delay is adjusted in the fourth delaycontroller 524 such that it is equal in propagation time to the secondIF signal. The first multiplexer 531 multiplexes the first IF signal andthe first local oscillation signal, and the second multiplexer 532multiplexes the second IF signal and the second local oscillationsignal. Respective multiplexed signals are outputted to the differentiallight-intensity modulator 550.

The differential light-intensity modulator 550 is a Mach-Zehnder typemodulator having two optical waveguides, as illustrated in FIG. 13, andis so constructed as to respectively apply voltage signals to electrodesprovided in correspondence with the optical waveguides, change therefractive index of each of the optical waveguides to provide thedifference in propagation time of light to optical signals, and thenmultiplex the optical signals. At this time, a bias voltage is appliedto each of the electrodes such that the difference in propagation timeof light passing through the two optical waveguides is converted intoπ/2 in terms of light phase, and the first and second local oscillationsignals are respectively applied to the electrodes in opposite phases.

Letting cos(ωL0t) be the local oscillation signal, and βL0 be both lightphase modulation indexes based on the first and second local oscillationsignals in the differential light-intensity modulator 550, the opticalsignal (electric field representation: EEMi(t)) outputted from thedifferential light-intensity modulator 550 is expressed by the followingequation (3) when no IF signal is inputted:

$\begin{matrix}\begin{matrix}{{{EEMi}(t)}\; = {\left. \sqrt{}\left\lbrack \begin{matrix}\left\{ {1 + {J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;\cos\;\left( {{\omega L}\; 0\; t} \right)}} \right\} \\{\;\left\{ {1 + {m\;\cos\;\left( {\omega\;\left( {t\; - \;{\tau\; 1}} \right)} \right)}} \right\}}\end{matrix}\; \right\rbrack \right.*}} \\{\sin\left\lbrack {{\omega\; 0\; t} + {\beta\; 1\;\sin\;\left( {\omega\; t} \right)}} \right\rbrack} \\{\;{= {\left. \sqrt{}\begin{bmatrix}{1 + {m\;\cos\;\left( {\omega\;\left( {t\; - \;{\tau\; 1}} \right)} \right)} +} \\{{J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;\cos\;\left( {\omega\; L\; 0\; t} \right)} +} \\{m\; J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;{\cos\left( {\omega\; L\; 0\; t} \right)}\;{\cos\left( {\omega\left( {t - {\tau\; 1}} \right)} \right)}}\end{bmatrix} \right.\;*}}} \\{\sin\left\lbrack {{\omega\; 0\; t} + {\beta\; 1\sin\left( {\omega\; t} \right)}} \right\rbrack}\end{matrix} & (3)\end{matrix}$

From the foregoing equation (3), it is found that the optical signaloutputted from the differential light-intensity modulator 550 has alight-intensity modulation component obtained by converting a modulationfrequency (ω) by the electrical-optical converter 540 by a modulationfrequency (ωL0) by the differential light-intensity modulator 550.

Then consider a case where only IF signals are inputted to thedifferential light-intensity modulator 550.

In this case, the first and second IF signals are respectively appliedto the electrodes of the differential light-intensity modulator 550 inthe same phase. Letting τ1 be a time period elapsed from the time whenthe third IF signal is outputted from the three-branching portion 510until it propagates to the differential light-intensity modulator 550after being converted into an optical signal in the electrical-opticalconverter 540, and τ2 be both time periods respectively elapsed from thetime when the first and second electric signals are outputted from thethree-branching portion 510 until they propagate to modulate the opticalsignals in the differential light-intensity modulator 550, and β2 belight phase modulation indexes based on the first and second IF signalsin the differential light intensity modulator 550, the optical signal(electric field representation: EEMp(t)) outputted from the differentiallight-intensity modulator 550 is expressed by the following equation (4)when no local oscillation signal is inputted.EEMp(t)=√{square root over ( )}[1+m cos(ω(t−τ1))] sin [ω0t+β1 sin(ω(t−τ1))+β2 cos [ω(t−τ2)+π/2]]  (4)

In the first and second delay controllers 521 and 522, when the amountof delay is adjusted such that τ2=τ1, the foregoing equation (4) ischanged into the following equation (5):EEMp(t)=√{square root over ( )}[1+m cos(ω(t−τ1))] sin[ω0t+(β1−β2)*sin(ω(t−τ1))]  (5)

From the foregoing equation (5), the light frequency modulation index β1caused by direct modulation can be decreased to β1−β2 using doublemodulation by the differential light-intensity modulator 550. Further,the frequency modulation component of the optical signal can becompletely removed by making the phase modulation index β2 equal to β1.

As described above, when both the local oscillation signal and the IFsignal are inputted to the differential light-intensity modulator 550,the optical signal (electric field representation: EEM(t)) outputtedfrom the output terminal 54 is expressed by the following equation (6)when β2=β1:

$\begin{matrix}\begin{matrix}{{{EEM}(t)}\; = {\left. \sqrt{}\begin{bmatrix}{\left\{ {1 + {J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;\cos\;\left( {{\omega L}\; 0\; t} \right)}} \right\}\;} \\\left\{ {1 + {m\;\cos\;\left( {\omega\;\left( {t\; - {\tau\; 1}} \right)} \right)}} \right\}\end{bmatrix} \right.*}} \\{\sin\left( {\omega\; 0\; t} \right)} \\{\;{= \left. \sqrt{}\left\lbrack \left\{ {1 + {m\;\cos\;\left( {\omega\;\left( {t\; - \;{\tau\; 1}} \right)} \right)}\; + \;{J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;\cos\;\left( {\omega\; L\; 0\; t} \right)} +}\; \right. \right. \right.}} \\{\left. {m\; J\; 1\;\left( {2\;\beta\; L\; 0} \right)\;{\cos\left( {\omega\; L\; 0\; t} \right)}\;{\cos\left( {\omega\left( {t - {\tau\; 1}} \right)} \right)}} \right\rbrack{\sin\left( {\omega\; 0\; t} \right)}}\end{matrix} & (6)\end{matrix}$

As apparent from the foregoing equation (6), a light frequencymodulation component caused in the electrical-optical converter 540 isremoved from the optical signal outputted from the differentiallight-intensity modulator 550 and at the same time, the optical signalhas a light-intensity modulation component obtained byfrequency-converting the modulation frequency (ω) by theelectrical-optical converter 540 by the modulation frequency (ωL0) bythe differential light-intensity modulator 550.

FIG. 14 illustrates an example in which a light frequency modulationcomponent caused by direct modulation is actually suppressed by externalmodulation. Illustrated in (a) of FIG. 14 is a light spectrum with thelight frequency modulation component caused by the direct modulation,while illustrated in (b) of FIG. 14 is a light spectrum with the lightfrequency modulation component canceled by the external modulation. InFIG. 14, no local oscillation signal is inputted to the differentiallight intensity modulator 550. From FIGS. 14( a) and 14(b), it can beconfirmed that the light frequency modulation component can besuppressed by using the high frequency optical transmitter according tothe seventh embodiment.

Description was made of an example in which the same local oscillationsignals having a phase difference of 180° are respectively inputted totwo local oscillation input terminals 52 and 53. At this time, the lightspectrum is an optical double-sideband (DSB) signal having upper andlower double-sidebands, as shown in (a) of FIG. 15. Generally, theoptical fiber has such wavelength dispersion characteristics that itvaries in group speed depending on the light wavelength (the lightfrequency). When the optical DSB signal is transmitted, therefore, thegroup speeds of the upper sideband and the lower sideband do notcoincide with each other, and a phase difference occurs in electricsignal components respectively obtained as beat components of the uppersideband and a light carrier and the lower sideband and a light carrierat the time of square detection by the optical receiver. Particularly atthe time of long-distance transmission, both the electric signals may becanceled upon being opposite in phase.

Examples of a method of avoiding the phenomenon include an opticaldouble-sideband (SSB) modulation scheme with only a single-sideband, asshown in (b) of FIG. 15, and an optical double-sideband (DSB-SC)modulation scheme with a light carrier suppressed, as shown in (c) ofFIG. 15. In the above-mentioned configuration of the seventh embodiment,the local oscillation signals having a phase difference 90° arerespectively inputted to first and second local oscillation inputterminals 52 and 53, thereby making it possible to easily realizeoptical SSB modulation. Further, a bias voltage is applied such that thedifference in propagation time of light passing through two opticalwaveguides in the differential light-intensity modulator 550 is π interms of light phase, and local oscillation signals having a phasedifference 180° are respectively inputted to the first and second localoscillation input terminals 52 and 53, thereby making it possible toeasily realize optical DSB-SC modulation. In the case of the optical DSBmodulation and the optical DSB-SC modulation, the local oscillationsignal may be inputted to only one of the two terminals in thedifferential light-intensity modulator 550, in which case the sameeffect is obtained.

As described above, according to the high-frequency optical transmitteraccording to the seventh embodiment of the present invention, thedifferential light-intensity modulator 550 is caused to perform a lightphase modulating operation, thereby making it possible to cancel thelight frequency modulation component caused at the time of directmodulation using the IF signal and at the same time, frequency-convertthe electric signal into the high frequency signal by thelight-intensity modulating operation using the local oscillation signal.Consequently, the differential light-intensity modulator 550 can havetwo functions, that is, an optical frequency conversion function and afunction of canceling frequency modulation, thereby making it possibleto obtain good transmission characteristics even when the high frequencysignal is transmitted a long distance by an optical fiber havingdispersion characteristics.

Eighth Embodiment

FIG. 16 is a block diagram showing the configuration of a high frequencyoptical transmitter according to an eighth embodiment of the presentinvention. In FIG. 16, the high frequency optical transmitter accordingto the eighth embodiment includes a two-branching portion 575, anelectrical-optical converter 540, a delay controller 525, a localoscillation signal source 560, and a phase modulator 581 and anintensity modulator 582 constituting an integrated modulator 580. Theoperation of the high frequency optical transmitter according to theeighth embodiment of the present invention will be described.

An IF signal inputted from an IF input terminal 51 is branched intofirst and second IF signals having a phase difference of 90°therebetween in the two-branching portion 575. The first IF signal andthe second IF signal are respectively inputted to the electrical-opticalconverter 540 and the delay controller 525. The first IF signal isconverted into an optical signal by direct modulation in theelectrical-optical converter 540, and the optical signal is outputted tothe phase modulator 581. The second IF signal is inputted to an opticalwaveguide of the phase modulator 581 through the delay controller 525. Atime period τ3 elapsed from the time when the first IF signal isoutputted from the two-branching portion 575 until it propagates to thephase modulator 581 after being converted into the optical signal in theelectrical-optical converter 540 and a time period τ4 elapsed from thetime when the second IF signal is outputted from the two-branchingportion 575 until it is modulated in the phase modulator 581 are madeequal to each other, thereby making it possible to reduce a lightfrequency modulation component caused at the time of direct modulation.

Generally, the amount of transmission delay is found by measuring thelevel of the inputted signal and the level of the received signal. In acase where the electric signal is phase-modulated by the phase modulator581, even if the optical signal is converted into the electric signal onthe side of light receiving, no electric signal component is obtained.Accordingly, the amount of transmission delay cannot be measured.

Therefore, the integrated modulator 580 constructed by integrating thephase modulator 581 and the intensity modulator 582 is used to measurean amount of transmission delay τ4′ in a case where an IF signal isinputted to the intensity modulator 582 and is transmitted by intensitymodulation. On the basis of the results, the amount of delay of thedelay controller 525 is first coarsely adjusted such that τ3=τ4′.Thereafter, the amount of delay of the delay controller 525 may beprecisely adjusted such that the light frequency modulation component isthe minimum by inputting the IF signal which has been inputted to theintensity modulator 582 again to the phase modulator 581 to which the IFsignal is to be inherently inputted, and measuring a light spectrum ofthe IF signal which has been subjected to direct modulation in theelectrical-optical converter 540 and phase modulation in the phasemodulator 581 using a light heterodyne method, for example.

The eighth embodiment is superior to the seventh embodiment in that aloss to the local oscillation signal may be small because it can bedirectly applied to the intensity modulator 582 without passing througha multiplexer or the like.

As described above, in the high frequency optical transmitter accordingto the eighth embodiment of the present invention, the phase modulator581 and the intensity modulator 582 are integrated in the integratedmodulator 580, thereby making it possible to easily make delayadjustment for canceling the light frequency modulation component.Further, a light phase modulating operation for directly inputting theIF signal and the local oscillation signal, respectively, to the phasemodulator and the intensity modulator without multiplexing the signalsto suppress a light frequency component caused at the time of directmodulation and a light-intensity modulating operation for frequencyconversion using the local oscillation signal are performed, therebymaking it possible to reduce a loss to each of the signals and toperform optical modulation more efficiently.

Although in the above-mentioned seventh and eighth embodiments,description was made of a case where the high frequency opticaltransmitter is applied to such a configuration that information istransmitted in a downstream direction from the center station to thesubscriber terminals, the high frequency optical transmitter is alsosimilarly applicable to such a configuration that information istransmitted in an upstream direction from the subscriber terminals tothe center station 100.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. An optical transmission system comprising: a center station; and aplurality of radio base stations connected to the center station with aplurality of optical fibers, the plurality of radio base stationscovering different service areas, wherein said center station includes:a light source for outputting predetermined light; a local oscillationsignal source for outputting a predetermined local oscillation signal;an external modulation portion for intensity-modulating the lightoutputted from said light source using the local oscillation signaloutputted from said local oscillation signal source; an opticalbranching portion for branching an optical signal obtained by theintensity-modulating in said external modulation portion into aplurality of optical signals whose number corresponds to a number ofsaid plurality of radio base stations; and a plurality of IF modulationportions, which are provided so as to correspond to said plurality ofradio base stations, each for intensity-modulating a respective one ofthe plurality of optical signals obtained by the branching in saidoptical branching portion using an inputted electric signal, andoutputting the intensity-modulated optical signal to a respective one ofthe plurality of optical fibers, and wherein the electric signalinputted to each of the plurality of IF modulation portions is one ormore IF signals in which one or more baseband signals are modulated bydifferent predetermined intermediate frequencies respectively, whereineach of said plurality of IF modulation portions intensity-modulates therespective optical signal such that only an IF signal component used ina service area covered by a corresponding radio base station istransmitted, and wherein said plurality of radio base stations eachincludes: an antenna portion for transmitting and receiving radiosignals to a subscriber terminal; and at least an optical-electricalconversion portion for converting the corresponding one of the opticalsignals transmitted through the plurality of optical fibers into anelectric signal in a radio frequency band, and feeding the convertedelectric signal to said antenna portion without passing the convertedelectric signal through a filter.